U.S. Pat. No. 5,386,253 to Fielding, incorporated herein in its entirety by this reference, discusses exemplary projection systems utilizing one or more spatial light modulators (SLMs). As noted in the Fielding patent:                Spatial light modulator devices include so-called “active matrix” devices, comprising an array of light modulating elements, or “light valves,” each of which is controllable by a control signal (usually an electrical signal) to controllably reflect or transmit light in accordance with the control signal. A liquid crystal array is one example of an active matrix device; another example is the deformable mirror device (DMD) developed by Texas Instruments . . .See Fielding, col. 1, II. 13–21. Of course, yet other types of light “engines,” or sources, and projectors exist, and various of them may be used in connection with the inventions described herein.        
Regardless of the type of projector used, audiences frequently desire to see images high in detail and richness and low in objectionable artifacts. High resolution and image quality in particular facilitates suspension of disbelief of an audience as to the reality of the projected images. Such quality indeed often is an important factor in the overall success of the motion picture viewing experience among today's audiences.
Producing these high-resolution images is not without added cost, however. Imax Corporation, for example, the intended assignee of this application, utilizes not only specialized cameras and projectors, but also seventy millimeter, fifteen perforation film to increase the resolution and quality of projected images. Conventional electronic projectors (and especially those utilizing SLMs), by contrast, generally cannot supply equivalent resolution in projected images. As well, such electronic projectors frequently fail to furnish the dynamic range and overall brightness of images provided by large-format films. They nonetheless may desirably (or necessarily) be employed to display non-film-based images such as (but not limited to) computer-generated graphics or material captured with electronic cameras.
In order to achieve the desired resolution, conventional electronic projection systems have employed “tiling” techniques. Tiling involves the use of multiple projection displays of sub-images that are displayed adjacent to each other to form a composite image. The use of multiple projection displays allows for greater resolution than is available with a single projection display. The sub-images can be blended inside a single projector or if multiple projectors are used, the sub-images are blended on the screen. For example, when two projectors are used one projector projects a first sub-image on a screen. A second projector projects a second sub-image on a screen. The first and second projectors are positioned such that the first and second sub-images are projected onto a screen adjacent to each other.
It is difficult to align the projectors exactly and therefore undesirable seams between the first and second sub-images are often apparent to the viewer. To improve the appearance and continuity of the composite image, the first and second projectors are conventionally positioned such that the first image slightly overlaps the second image. Mere overlapping of sub-images typically is insufficient, however, as the additive intensity of the images in the regions of overlap in some scenes likewise may be noticeable to audiences. General methods of reducing brightness in these regions typically include adjusting the images either electronically or optically; the latter method is usually implemented using an opaque or reflective element placed in the beam of light. Thus, invisible seaming of multiple projectors requires careful matching of the displays at the seam area(s), both geometrically and photometrically.
U.S. Pat. No. 6,017,123 to Bleha et al., incorporated herein in its entirety by reference, discloses one system and method for blending sub-images. FIG. 13 of Bleha et al. shows the use of filters and masks, located between the projection lens and the screen, to blend the sub-images. The mask used by Bleha is a knife edged mask. Introducing simple knife edge masks into the optical path, in front of the projection lens, generally produces a guassian blurred edge, which when combined with a complimentary edge on the other projection lens, and precisely aligned so that the sum of their black levels remains as constant as possible throughout the overlapped seam area, will produce a very good uniform black level throughout the image, including the seam area. These knife edges block the image from the respective projectors outside of the overlap region, eliminating stray light that often surrounds images from most projection technologies. However, simple straight knife edges introduce a luminance “ringing,” whereby the edge has a slightly sinusoidal pattern to the decay from full luminance to system black. This “ringing” is generally attributed to artifacts arising from integrating bars utilized in projection system lamphouses, or from simple edge diffraction effects, and leads to an obvious line pattern along the seam visible in gray scale and peak white images. Other traditional methods of butt seaming or overlapping images have typically failed to achieve sufficient precision in matching to make the seam truly invisible for a wide range of image content or projection geometries.
Therefore, techniques and equipment for blending multiple projection displays to form a single display with solutions to the geometric and photometric issues is desirable.